The present invention relates to a dual-resonance antenna that can be used in two mutually separated frequency bands employed in cellular phones or handyphones (PHS: personal handyphone system).
The number of cellular phone or PHS subscribers increases from year to year, and because of such an increase in the number of subscribers, the employed frequency is insufficient. When the employed frequency is insufficient because of such an increase in the number of subscribers, two frequency bands are allocated: a frequency band that can be used almost everywhere as the frequency band of cellular phones and a frequency band that can be used in cities. For example, in Europe, cellular phones of a GSM system with a 900 MHz band can be used everywhere, and also cellular phones of DCS system with a 1.8 GHz can be used in cities in order to compensate for the utilized frequency insufficiency. For a cellular phone to be thus used in two frequency bands, it has to be made suitable for operation in two frequency bands. Thus, it has to contain wireless circuitry for each frequency band of the two frequency bands and to be provided with a dual-resonance antenna operating in two frequency bands.
A dual-resonance antenna shown in FIG. 9 has been suggested as the dual-resonance antenna of such type. This dual-resonance antenna comprises a helically wound coil 121 and a connection member 122 obtained by bending the upper end portion of the coil 121 downward and passing it inside the coil 121 almost along the central axis of coil 121. Power is fed from a feeder 124 to the end portion of the connection member 122.
An equivalent circuit of dual-resonance antenna 114 shown in FIG. 9 is shown in FIG. 10. As shown in FIG. 10, the coil 121 and connection member 122 passing inside the coil 121 are high-frequency coupled, a floating capacitance is generated, and a parallel resonant circuit comprising an inductor L101 and a capacitor C101 is equivalently formed. An equivalent element 125 is equivalently formed above this parallel resonant circuit, and an equivalent element 126 is equivalently formed between the parallel resonant circuit and feeder 124. The equivalent element 125 is formed by the coil 121, and the equivalent element 126 is formed by the connection member 122.
In such a dual-resonance antenna 114, the coil 121 together with the connection member 122 operate as an antenna in a low-frequency band (first frequency band), the parallel resonant circuit is caused to operate as a trap in a high-frequency band (second frequency band), and the connection member 122 operates as an antenna at a high frequency. Thus, the dual-resonance antenna 114 operates at two frequency bands, namely first and second frequency bands.
In such a dual-resonance antenna, the antenna operating in a high-frequency band is formed by a linear connection member 122. Therefore, the length of connection member 122 has to correspond to the frequency of the second frequency band. The problem, however, is that if the length of connection member 122 is selected so as to correspond to the frequency of the second frequency band, the length of dual-resonance antenna 114 is increased and the size of antenna is difficult to reduce. For this reason, the size reduction of dual-resonance antenna 114 operating in two frequency bands, first frequency band and second frequency band, was attained by decreasing the length of connection member 122 to a level less than that essentially required and connecting a matching circuit with a dual-resonance characteristic. FIG. 11 shows a VSWR characteristic of dual-resonance antenna 114 with a total length reduced to about 20 mm, which has such a matching circuit connected thereto. In the VSWR characteristic shown in FIG. 11, frequency is plotted against the abscissa, a 900 MHz band (890-960 MHz) in the GSM (global system for mobile communication) is a first frequency band, and a 1.7 GHz band (1710-1880 MHz) in a DCS (Digital Cellular System) is a second frequency band. As shown in FIG. 11, the worst value of VSWR in the first frequency band is 3.1, the worst value of VSWR in the second frequency band is 2.7, and good VSWR is not obtained.
Furthermore, in the VSWR characteristic shown in FIG. 11, the matching circuit shown in FIG. 12 is connected between the dual-resonance antenna 114 and feeder 124. In order to obtain a dual-resonance characteristic, this matching circuit is composed by connecting a second inductor L112 and a third inductor L113 in series, connecting a capacitor C111 between the ground and the connection point of the second inductor L112 and the third inductor L113, and connecting the first inductor L111 between the ground and the initial end of the second inductor L112. In this case, the first inductor L111 is about 15 nH, the second inductor L112 and third inductor L113 are about 4.7 nH, and the capacitor C111 is about 2 pF. Thus, the problem was that the dual-resonance antenna 114 required a complex matching circuit using four or more elements.
Accordingly, it is an object of the present invention to provide a dual-resonance antenna that can be miniaturized without degrading the electric characteristics and that employs a simple matching circuit.
In order to attain this object, the dual-resonance antenna in accordance with the present invention comprises a first coil, a connection member obtained by bending an end portion of the first coil and passing it along almost the central axis inside the first coil, and a second coil connected to the end portion of the connection member.
Furthermore, in the dual-resonance antenna in accordance with the present invention, a first reactance element for matching may be connected in series between the end portion of said second coil and a feeder, and a second reactance element for matching may be connected between the end portion of said second coil and the ground.
Moreover, in the dual-resonance antenna according to the present invention described in the above, a xcfx80-type matching circuit or a T-type matching circuit composed of a third reactance element may be connected between the end portion of the second coil and a feeder.
In accordance with the present invention, since the second coil is connected to the end portion of the connector member passed along almost the central axis inside the first coil, the total length of the dual-resonance antenna can be reduced and the antenna can be miniaturized. Furthermore, despite the size reduction, the second coil with an inherently required length can be used. As a result, a dual-resonance antenna with good electric characteristics can be obtained. Furthermore, since a matching circuit providing a dual-resonance characteristic is not required, a simple circuit with a small number of components can be used as the matching circuit for feeding the dual-resonance antenna.